The present invention relates to a microcapsule for the pressure-sensitive recording paper and a process for producing the same. More precisely, the present invention relates to a microcapsule for pressure-sensitive recording paper, wherein the microcapsule contains a solution of a colour-former and has membranous wall of resin formed by polycondensation of at least two prepolymers selected from a melamine-formaldehyde prepolymer, a thiourea-formaldehyde prepolymer and a melamine-thiourea-formaldehyde prepolymer, or a prepolymer of a melamine-thiourea-formaldehyde prepolymer with a water-soluble cationic urea resin, and to a process for producing a microcapsule for the pressure-sensitive recording paper, wherein a water-soluble cationic urea resin and a prepolymer selected from the group consisting of a mixture of a melamine-formaldehyde prepolymer and a thiourea-formaldehyde prepolymer, a melamine-thiourea-formaldehyde prepolymer and a mixture of a melamine-formaldehyde prepolymer, a thiourea-formaldehyde prepolymer and a melamine-thiourea-formaldehyde prepolymer are polycondensed on the surface of dispersed droplets of a colour-former solution in the presence of an anionic surfactant by adding an acidic catalyst while causing a complex-coacervation between the water-soluble cationic urea resin and the anionic surfactant.
The commonly used pressure-sensitive recording paper is prepared by laminating a CB paper having its under surface coated with a plurality of microcapsules containing a solution of a leuco-type dyestuff (colour former) as a core substance with a CF paper coated with a developer consisting of an acid clay or a resin exhibiting acidity, and the thus prepared pressure-sensitive recording paper is used in a manner wherein the microcapsules are broken by a pressure of hand-writing or typewriting pressure to bring the colour former and the developer into mutual contact thus obtaining the colour-development and recording by the developed colour.
Recently, with the improvement of office efficiency, particularly with the development and the spread of office computers, the pressure-sensitive recording paper has come to be used more and more in various fields, and as a result, its durability under severe use conditions is demanded. Accordingly, a microcapsule having excellent moisture-resistance, heat-resistance and light-fastness has been demanded for a pressure-sensitive recording paper. In addition, in the preparation of the pressure-sensitive recording paper, the microcapsule excellent in solvent-resistance is also demanded.
In this connection, the conventional pressure-sensitive recording paper has been prepared by coating a paper with an aqueous slurry-like dispersion of microcapsules, the dispersion containing a water-soluble binder and some additives. However, in the case where water is used as a dispersion medium, it takes much time to dry the coated paper and the product lacks dimensional stability because of the occurrence of wrinkles on drying.
In order to overcome these problems and improve the productivity of the pressure-sensitive recording paper, the use of a quick drying dispersion medium as the medium for dispersing microcapsules in coating the paper is considered. As a dispersion medium for that purpose, an organic solvent used as a solvent for a printing ink, for instance, alcohols such as isopropyl alcohol and ethanol, esters such as ethyl acetate, ketones such as methyl ethyl ketone, hydrocarbons such as kerosene, toluene and xylene and vegetable oils such as linseed oil and castor oil is suitable.
However, any microcapsule which has been practically used or proposed for use in the pressure-sensitive recording paper cannot remain in the organic solvent in a stable state. In the case where the microcapsule which can exist in such a solvent in stable state is actually prepared, it is possible not only to reduce the time for drying the paper coated with the slurry-like dispersion of microcapsule containing a colour former therewithin in the industrial production of the pressure-sensitive recording paper resulting in a large improvement of productivity but also there will be no fear of causing wrinkles. In addition, the production of partially pressure-sensitive recording paper is easily carried out applying spot printing. Accordingly, a microcapsule which has excellent solvent-resistance is demanded for the pressure-sensitive recording paper.
The term "solvent-resistance" herein means the stability of a microcapsule in an organic solvent.
The membranous wall of microcapsule used for almost all the pressure-sensitive recording papers which are practically used at present is made of gelatine as the main material (hereinafter referred to as gelatine-capsule). Gelatine-capsule has the problem of having poor moisture-resistance, light-fastness and solvent-resistance.
On the other hand, as a substitute for the gelatine-capsule various microcapsules having a hydrophobic high polymer as their material for membranous wall have been proposed, for instance, a urea-formaldehyde resin, a melamine-formaldehyde resin, a polyamide and a polyurethane. These materials are capsulated by in situ polymerization or interface-polymerization. However, although the hitherto proposed microcapsules are improved to a certain extent in moisture-resistance, light-fastness, etc., they are still unsatisfactory as the microcapsule for the pressure-sensitive recording paper, and are poor in solvent-resistance. For instance, as a process for forming the microcapsule solely from a prepolymer consisting of melamine, thiourea and formaldehyde, the process has been proposed wherein a urea-formaldehyde compound as a reactive surfactant or a substance which is derived from melamine-formaldehyde compound and has both hydrophobic group(s) and hydrophilic group(s) is used in combination with the prepolymer(refer to Japanese patent application Laying Open No. 7313/1971). Although the proposed method has overcome some of the problems in preparing gelatine-capsules capsulation is still only possible from a solution of low concentration and the thus prepared microcapsule is poor in stability in the organic solvent as will be described. However, their moisture-resistance and light-fastness, etc. have been improved to a certain extent.
In consideration of these situations, the inventors of the present invention have studied the process for preparing a microcapsule which has excellent solvent-resistance for the pressure-sensitive recording paper while retaining the characteristics of the microcapsule having melamine-thiourea-formaldehyde resin as the membranous wall, that is, the hydrophobicity thereof and availability of raw materials thereof at a low price, and have accomplished the present invention.
The object of the present invention is to provide, microcapsules having the shell thereof mainly composed of melamine-thiourea-formaldehyde resin and suitable for preparing pressure-sensitive recording paper which is far superior in solvent-resistance to the conventional pressure-sensitive recording paper.